U.S. patent number 5,218,294 [Application Number 07/860,069] was granted by the patent office on 1993-06-08 for contactless test method for testing printed circuit boards.
This patent grant is currently assigned to Advanced Test Technologies Inc.. Invention is credited to Jacob Soiferman.
United States Patent |
5,218,294 |
Soiferman |
June 8, 1993 |
Contactless test method for testing printed circuit boards
Abstract
This application describes a novel method and its implementation
for testing unpopulated and populated electronic printed circuit
boards (PCBs). This method can be used to develop a new contactless
test system (CTS). While eliminating drawbacks of existing test
systems, this method measures electromagnetic near field
distribution in the vicinity of a PCB, contactlessly, by using
suitable sensors (possibly printed near field planar antennas) and
sensitive measuring and processing devices. The electromagnetic
fields (EMF) are generated by the distribution of charges and
currents on paths and elements of the board under test (BUT).
Therefore, accurate and repeatable measurements of these fields
produce a specific pattern for the BUT. Such a pattern is then
compared to a known pattern for the same type of board to determine
whether the BUT is faulty or nonfaulty.
Inventors: |
Soiferman; Jacob (Winnipeg,
CA) |
Assignee: |
Advanced Test Technologies Inc.
(Weyburn, CA)
|
Family
ID: |
24580470 |
Appl.
No.: |
07/860,069 |
Filed: |
March 30, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
643356 |
Jan 22, 1991 |
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Current U.S.
Class: |
324/763.01;
324/95 |
Current CPC
Class: |
G01R
31/309 (20130101); G01R 31/315 (20130101) |
Current International
Class: |
G01R
31/309 (20060101); G01R 31/315 (20060101); G01R
31/28 (20060101); G01R 031/28 () |
Field of
Search: |
;324/95,536,158R,537,538,201,244,83.1,500 ;455/67 ;358/101,106
;340/600 ;343/703 ;250/341,334 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Vinh
Attorney, Agent or Firm: Battison; Adrian D. Ade; Stanley G.
Thrift; Murray E.
Parent Case Text
This application is a continuation application of application Ser.
No. 643,356, filed Jan. 22nd, 1991 now abandoned.
Claims
I claim:
1. A method for testing an unpopulated circuit board including a
test arrangement of electrically conductive paths, parts, and
surfaces whose electrical and physical continuity and conformance
to a known standard of a sample arrangement of unpopulated circuit
board is to be verified, the sample arrangement having a
predetermined required structure for carrying out a predetermined
required function, the method comprising providing said known
sample arrangement having said required structure and said required
function, applying to the sample arrangement an electrical signal,
the signal being independent of the predetermined function,
providing an array of non-contact sensors for detecting the
electromagnetic near field distribution generated by the sample
arrangement in response to the signal, using the array to create a
sample pattern representative of the electromagnetic near field
distribution of the sample arrangement, applying to said test
arrangement said electrical signal, using said array to create a
test pattern representative of the electromagnetic near field
distribution of the test arrangement, making a comparison of the
test pattern with said sample pattern of said known sample
arrangement and determining from said comparison whether the test
arrangement is in conformance to the known standard, the
non-contact sensors of the array each comprising a planar printed
spiral loop antenna.
Description
TECHNICAL FIELD
This invention is in the general field of test and measurement of
the quality, shape, and/or dimensions of conducting paths, pads,
traces, and electronic components formed or placed on the surface
or on intermediate layers of a printed circuit board (PCB), ceramic
substrate, or like items.
This invention makes available a new and improved method and system
for the automatic testing and measurement of unpopulated and/or
populated printed circuit boards. This invention enables the
detection of electrical defects of components, shorts,
discontinuities, and tolerance problems on the board under test
(BUT).
BACKGROUND PRIOR ART
During the manufacture or subsequent handling of PCBs, defects such
as discontinuities (cracks) or unwanted continuities (shorts) may
develop in or between circuit pathways and electronic components.
It is necessary to do automated testing of PCBs both for
manufacturing and maintenance purposes.
Testing of PCBs is becoming increasingly difficult and more
expensive as the use of surface mount technology increases and as
integrated circuits and PCBs become more complex and operate at
higher frequencies. Conventional techniques for automated PCB
testing are based on the idea of applying signals through a set of
test pins and measuring output signals at other test pins. This
method requires tight mechanical tolerances for the board layout,
easily accessible test points, and restricts the frequency band at
which a board can be tested (most of the test systems are limited
to 100 MHz). The novel method presented here does not have these
constraints because of its contactless nature. Another factor
separating existing test techniques from this invention, is the
contactless test system (CTS) universality. The CTS does not need
the custom setup of test pins and test patterns for the BUT, which
make presently used test systems expensive and inaccessible to some
complex circuit boards. Therefore, the applied CTS offers
substantial advantages over existing test methods which utilize
electrical contact.
Non-contact probes have been used for measurements on high
frequency microwave circuits. However, at frequencies below 1 GHz,
the test is difficult due to the high bandwidth of the probes. Most
recent advances in the test equipment industry have resulted in
devices for the evaluation of electromagnetic compatibility (EMC)
of PCB assemblies. However, these devices, in their present forms,
provide only information about electromagnetic interference caused
by the BUT, and can not be used for providing detailed information
about the performance of the BUT. This invention is targeted at
measuring detailed EMF for testing the quality and functionality of
the BUT.
SUMMARY OF INVENTION
This present invention provides a novel method for testing the
quality, shape and/or dimensions of conducting paths, pads, traces,
and electronic components formed or placed on the surface or on
intermediate layers of a printed circuit board or ceramic
substrate. The invention eliminates drawbacks in existing test
methods and therefore offers a viable method for the automated
contactless performance testing of printed circuit boards
(unpopulated and populated). The invented method measures
electromagnetic near field distribution in the vicinity of a PCB,
contactlessly, for performing the test described above. The
electromagnetic fields (EMF) are generated by the distribution of
charges and currents on paths and elements of the board under test
(BUT). Accurate and repeatable measurements of these fields produce
a specific pattern for the BUT. Such a pattern is then compared to
a known pattern for the same type of board to determine whether the
BUT is faulty or non-faulty.
Thus firstly a known standard of an unpopulated circuit board
having a required structure and a required function is selected and
a signal is applied to the sample arrangement, the signal being
independent of the predetermined function of the circuit board. The
electromagnetic near-field distribution generated by the sample is
then detected using an array of non-contact sensors and the array
is used to create a sample pattern representative of the
electromagnetic near-field distribution sample arrangement.
Subsequently the near-field distribution of a test arrangement is
similarly detected and a pattern created therefrom. A comparison is
then made between the sample pattern and the test pattern to
determine whether the test arrangement is in conformance with the
known standard.
The term unpopulated circuit board is intended herein to comprise a
circuit board prior to the application thereof of the electronic
components.
The above described method can be used to develop a new contactless
test system (CTS). The CTS consists of suitable sensors (possibly
printed near field planar antennas, monopole antennas, fiberoptic
sensors or like items capable of measuring a wide range of signals
up to 1 GHz in frequency range), a sensor control unit, a signal
source, sensitive measuring and signal processing devices, a
central computer workstation, and a test platform onto which the
BUT is mounted. The sensor control unit controls the movement of
sensors (sensor array) and switches the measured signals to the
measuring device. The signal source provides signals, independent
of the functionality of any specific board, to energize the BUT.
The measurement and signal processing device can be a spectrum
analyzer or a network analyzer of wide range frequency bandwidth.
The central workstation controls the whole system by commanding
sensor movement and sensor switching, receiving the measured
results from the spectrum analyzer, and running the off-line fault
recognition procedure.
A complete scan of the BUT constitutes one plane of signals. The
plane of signals is made up of signals obtained from individual
sensors. The signal from each sensor is particularly sensitive to
the local electromagnetic field. Therefore, signals from a whole
plane of sensors can be processed to image the board's
electromagnetic signature. The objective of the signal processing
is to extract relevant features from the signals which represent
the characteristics of the BUT. These features are used to create
images representing the BUT signature. The fault recognition system
compares the image of the BUT with a known or desired image of an
identical, non-faulty PCB. Such comparisons result in a measure of
the difference between the non-faulty and faulty boards. If the
difference is larger than a predetermined threshold, the BUT is
diagnosed to be faulty. If the difference is smaller than the
threshold, the BUT is diagnosed to be non-faulty. Statistical
analysis can further result in more detailed information of faults
found, such as locations, types, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
The following figures accompany this application to provide a
better understanding of the concept of this invention. They reflect
implementation aspects of this invention. Each of the figures is
identified by a reference character, and wherein:
FIG. 1 is the hardware architecture of the CTS, illustrating
hardware components of the implemented system and their
interrelationships.
FIG. 2 is the software architecture of the CTS, illustrating
software components of the implemented system and their
interrelationships.
FIG. 3 illustrates a one-dimensional sensing array, reflecting the
concept of a plane sensing mechanism.
FIG. 4 illustrates one-dimensional images of a faulty and
non-faulty board.
FIG. 5 is a top view of the topology of a printed spiral
antenna.
FIG. 6 illustrates a one-dimensional sensing array similar to that
of FIG. 3 in which the energizing signal is applied by contact with
the board.
IMPLEMENTING THE INVENTION
As illustrated in FIG. 1, the implemented contactless test system
(CTS), for testing PCBs, consists of suitable sensors (a planar
sensing array) 10, a sensor control unit 11, a signal generator 12,
a spectrum analyzer or network analyzer 13, a central computer
workstation 14, and a test platform 15 onto which the BUT is
mounted.
The applied sensor is developed based on interrelations between
current, charge, and electric and magnetic field intensities
described by the Coloumb-Maxwell, Ampere-Maxwell and continuity
equations. They are of the printed planar loop antenna type capable
of measuring a wide range of signals up to 1 GHz in frequency
range.
For unpopulated BUTs, the signal generator 12 provides common
signals, independent of the functionality of any specific board, to
energize the BUT at a desired frequency through the power and
ground lines of the BUT (FIG. 4). The power and ground lines of the
BUT are usually distributed throughout the board, therefore,
through coupling, the whole plane of the BUT is energized. The
validity of this energizing method can be verified by comparing a
measured result from any location on the plane with the noise level
of the test environment. If the ratio between the measured result
and the noise level is larger than a predetermined signal to noise
ratio, then the validity of having energized the whole board is
verified. Other common energizing methods are also conceivable. For
instance, the BUT can be energized by near field active antenna
probes (a type similar to the sensors), connected to the signal
generator 12A (FIG. 3), and moving them in conjunction with the
sensing probes. This way, the board is guaranteed to be energized
because the energizing signal moves with the sensors. This
energizing method is more universal and contactless.
For populated BUTs, the energizing method should be modified so
that signals applied to the BUT will depend on the functionality of
the board.
The sensor control unit controls the movement of a sensor array
within the test plane and switches individual sensors, within the
array, to the measuring device. The switching circuit within the
sensor control unit is able to operate at the desired frequency
bandwidth.
The measurement and signal processing device can be a spectrum
analyzer or a network analyzer with a wide range frequency
bandwidth. The measuring device transforms a time-domain signal
from the sensor to its frequency-domain expression. The
frequency-domain expression is then integrated to produce a
characteristic for the specific sensor.
The central workstation controls the whole system: commanding
sensor movement and switching, receiving measured results from the
spectrum analyzer or network analyzer, and running the off-line
signal analysis and fault recognition procedures.
The test platform contains the BUT within a grounded metallic
enclosure 16 (FIG. 3). The purpose of this enclosure is to provide
electromagnetic shielding for the interior of the test
platform.
FIG. 2 illustrates the software procedures required for the CTS.
The sensor control procedure 20 controls the movement and switching
of the sensing array. The spectral analysis procedure 21 receives
the measured results from the spectrum analyzer or the network
analyzer and calculates, over specific frequency bandwidths, the
integration of the power spectral density (spectral analyzer mode)
or the phase shift (network analyzer mode) of the acquired signals
from the sensors. The calculated result constitutes pixel values
for an image representing each sensor element at particular
locations of the test plane. Such an image represents the
characteristic of the BUT. The characteristics can then be
displayed on the computer screen with the imaging procedure or
passed on to the fault recognition procedure to determine whether
the BUT is faulty or non-faulty. The fault recognition procedure is
based on a comparison between the image of the BUT and a known
image of a non-faulty board of the same type. Such a comparison
results in a measure of difference between the two boards. If the
difference is larger than a predetermined threshold, the BUT is
diagnosed to be faulty. If the difference is smaller than the
threshold, the BUT is diagnosed to be non-faulty. Further, regional
comparisons can result in information on the actual location of the
faults.
FIG. 3 illustrates the side sectional view of the test board with a
conducting path layout on the board and an array of sensors above
the test board within a grounded metallic enclosure 16. A sensor
array may be implemented with a stationary sensor array or a small
array of sensors moving across the test plane of the test
board.
FIG. 6 illustrates a side sectional view of the test board with a
conducting path layout on the board and an array of sensors above
the test board. In this arrangement, the energizing signal is
applied through a first conductor 12A of the board for connection
to the positive supply and a second conductor 12B for connection to
a ground return.
FIG. 4 illustrates the one dimensional view of the resultant images
obtained according to the above described procedures. The test
board is energized by a sinusoidal signal (5 volt peak to peak, at
a frequency above 10 MHz). Graph curve (1) is the result of a test
board without the introduced fault (a short). Graph curve (2) is
the result of a test board with the introduced fault (a short).
Because of the introduced short (curve (2)), the adjacent paths are
more closely coupled than without the short (curve (1)), therefore
they emit a higher signal intensity. The two image results can then
be compared to diagnose the fault.
Theoretical background for the solution of loop antennas and their
derivatives results from integrating the Maxwell-Faraday equation
over a loop area Sa and applying Stokes Theorem thus obtaining
Consider a printed rectangular spiral antenna-sensor whose top view
is depicted in FIG. 5. The sensor is exposed to the reactive (or
equivalently fringing) electric and magnetic near-fields
surrounding the PCB. The portion of the near-field coinciding with
the fringing field is commonly assumed to exist in the space
confined at the distance 1/.kappa.=.lambda./2.pi. from an
equivalent radiator. The sensor is placed inside a structure which
consists of metallic planes of large dimensions. The distance
between the planes is small as compared with the wavelength of the
upper frequency limit of the applicator. The purpose of this
structure is to provide the eddy current shielding of the interior
of the applicator. The Green's function of structure is completely
determined by this geometry of the contactless tester.
The fringing near-field performance of a single antenna sensor is
of interest in response to the standard fields produced by the
chosen radiators.
A printed loop antenna may be modeled as an uniformly
impedance-loaded loop. The boundary conditions satisfied on the
metallic surface S of the sensor are:
and
on the dielectric surface S.sub.d. Y.sub.s is the surface
admittance of the dielectric layer given by
Where .epsilon..sub.o is the substrate permittivity while
.mu..sub.o Equation (1) allows to find a general integral equation
for the zero phase-sequence current by the use of the generalized
Ohm's law
The second term on the right hand side represents interaction due
to reradiated field. The contour integral in (5) is defined for a
single loop. Therefore in order to apply (5) to the printed spiral
antenna, segmentation of a spiral into elementary loops is imposed.
Then for each loop-segment equation (5) is valid. The continuity of
current I.sub.o is applied at the separation of each constitutive
loop.
Equation (5) is not tractable yet for numerical solution due to the
fact both I.sub.o (l) and K(e,rar/x/ ) are unknown The Lorentz
reciprocity theorem applied to the fields inside the antenna gives
second condition which leads to the integral equation:
where E is a function of current K(x.sup.1) on the printed board.
Equations (5) and (6) are coupled integral equations for the
unknown currents. In order to solve the problem, it is natural to
use the rectangular coordinate system because of the geometry in
FIG. 5.
Having described the antenna characteristics, the signal received
from the antenna must be processed to image the BUT's
electromagnetic signature.
Denote a signal from each sensor as s.sub.ij (t) for i=l, . . . N
and j=l, . . . , M, where t is the time variable, N and M are the
size of the two dimensional sensor matrix. The pattern of s.sub.ij
(t) is changed by changes in the signature of the PCB under
test.
A signal from each sensor can also be represented in the frequency
spectral domain. Denoting Fourier transform ##EQU1## where R.sub.ij
(.omega.) and X.sub.ij (.omega.) represent real and imaginary
functions of the Fourier transform of signal s.sub.ij (t) which has
a frequency spectrum given by ##EQU2##
In the spectral domain, imaging of the PCB under test (processing
in the spatial domain) is obtained by integrating the spectra in
some specific bandwidths. The result of integration for a single
sensor is the pixel value (associated with a picture element). The
pixel value is computed for bandwidths where F.sub.ij (.omega.)
possesses high signal-to-noise ratio.
Denoting by K the number of significant bandwidths, a signature
image of a printed circuit board is defined for the k-th bandwidth
in the following form ##EQU3## where I.sub.k (ij) represents the
power spectrum image in k-th frequently band .omega..sub.k.sbsb.L
to .omega..sub.k.sbsb.U.
The phase angle .phi.(.omega.) may also be used as a measure of
pixel value in an image.
* * * * *